From the viewpoint of protection of the global environment, these days an increasing number of boiler-based heating apparatuses that use fossil fuel for the heating operation have come to be substituted with heat pump-based air-conditioning apparatuses that utilize air as heat source, even in cold districts.
The heat pump-based air-conditioning apparatus is capable of performing the heating operation more efficiently because, in addition to electrical inputs to a compressor, heat from the air can be utilized.
On the other hand, when the outdoor temperature drops, frost is formed on an outdoor heat exchanger serving as evaporator, and hence a defrosting operation has to be performed to melt the frost formed on the outdoor heat exchanger.
To defrost, the refrigeration cycle may be reversed. However, in this case, the heating of the room is suspended during the defrosting operation, and consequently comfort is impaired.
Thus, as one of methods to perform the heating operation even during the defrosting operation, a technique has been proposed that includes dividing the outdoor heat exchanger to cause a part of the divided outdoor heat exchangers to act as evaporator, and receiving heat from air in the evaporator thereby performing the heating operation while the other heat exchanger is performing the defrosting operation (see, for example, Patent Literature 1 and Patent Literature 2).
With the technique according to Patent Literature 1, the outdoor heat exchanger is divided into a plurality of parallel heat exchangers, and a part of high-temperature refrigerant discharged from the compressor is alternately supplied to each of the parallel heat exchangers to thereby alternately defrost the parallel heat exchangers. Thus, the heating operation can be continued without reversing the refrigeration cycle.
With the technique according to Patent Literature 2, the outdoor heat exchanger is divided into two parallel heat exchangers, namely an upper outdoor heat exchanger and a lower outdoor heat exchanger. When one of the heat exchangers is defrosted, a main circuit opening and closing mechanism, on the side of the inlet of the heat exchanger to be defrosted in the heating operation, is closed, and a bypass on-off valve of a bypass circuit, through which the refrigerant from the discharge pipe of the compressor flows to the inlet of the heat exchanger, is opened. Consequently, a part of the high-temperature refrigerant discharged from the compressor is made to flow into the heat exchanger to be defrosted, so that the defrosting and the heating can be performed at the same time. When one of the heat exchangers has been defrosted, the defrosting of the other heat exchanger is started. In addition, a hot pipe is interposed between an indoor heat exchanger and a depressurizing device, under the upper outdoor heat exchanger. The refrigerant flowing out of the outlet of the indoor heat exchanger is made to flow into the hot pipe when the defrosting and the heating are performed at the same time, to enhance the defrosting effect in the boundary between the upper outdoor heat exchanger and the lower outdoor heat exchanger, thus to prevent formation of root ice.